1. Field of the Invention
The present invention relates to a charged particle beam apparatus and a method for operating the same, especially the present invention relates to an electron microscope which may be operated in different modes of operation allowing to switch from a serial imaging mode to a parallel imaging mode and vice versa.
2. Description of the Related Art
A variety of methods have been used to examine microscopic surface structures of semiconductors. These have important applications in the field of semiconductor chip fabrication, where microscopic defects at a surface layer make the difference between a good or bad chip. Holes or vias in an intermediate insulating layer often provide a physical conduit for an electrical connection between two outer conducting layers. If one of these holes or vias becomes clogged, it will be impossible to establish this electrical connection and the whole chip may fail. Examination of microscopic defects on the surface of the semiconductor layers is necessary to ensure quality control of the chips.
Charged particle beams have several advantages over other mechanisms to examine samples. Light beams have an inherent resolution limit of about 100 nm to 200 nm, but electron beams can investigate feature sizes as small as a few nanometers. Electron beams are manipulated fairly easily with electrostatic and electromagnetic elements, and are easier to produce and manipulate than x-rays.
A variety of approaches involving charged particle beams have been utilized for examining surface structure. In scanning electron microscopy (SEM), a narrow beam of primary electrons is raster-scanned across the surface of a sample, also called the working plane. Primary electrons in the scanning beam cause the sample surface to emit secondary or backscattered electrons. Because the primary electrons in the beam of scanning electron microscopy are near a particular known electron energy there is a reduced corresponding charge build-up problem in SEM when compared to other inspection methods, and the surface of the sample remains substantially neutral. However, raster scanning a surface with scanning electron microscopy is slow because each pixel on the surface is collected sequentially. In this context, a pixel may be understood as the diameter of the primary beam on the surface of the sample. However, other definitions may apply more properly for different geometries. Moreover, a complex and expensive electron beam steering system is needed to control the beam pattern.
Another approach called Secondary Electron Emission Microscopy (SEEM) can be much faster than SEM because SEEM does not scan a narrow beam across the sample, but instead directs a relatively wide beam of electrons at the surface. To put this in numerical perspective, the spot size of the scanning beam in Scanning Electron Microscopy (SEM) is typically about 5 nm to 100 nm. The area illuminated by the incident beam in conventional Secondary Electron Emission Microscopy (SEEM) is about 1 mm to 10 mm. Thus, the area illuminated by the beam in conventional SEEM is on the order of 109 to 1012 times larger than in SEM. Accordingly, SEEM is able to look at a larger surface more rapidly than it is possible in SEM and, in this context, is therefore considered as a parallel imaging method. The primary electron energies in SEEM are close to the point used in SEM, i.e. about 0.5 to 1 keV. Such a SEEM device is described in U.S. Pat. No. 5,973,323 (Adler et al.).
The comparative speed advantage in SEEM, i.e. the maximum pixel rate, is limited mainly by the exposure time and the current density. The minimum exposure time that a beam must spend looking at a given image is determined by the acceptable Signal-to-Noise ratio of the image. The maximum current density is determined by such practical considerations as available gun brightness and possible sample damage, but also by electron-electron interaction. Because the focused beam of primary electrons in SEM must scan the beam across the entire surface to be inspected, the maximum practical pixel rate in Scanning Electron Microscopy is less than or equal to 100 million pixels/second (100 MHz). In Secondary Electron Emission Microscopy (SEEM), a large two-dimensional area of the sample is imaged in parallel without the need for scanning. The maximum pixel rate in SEEM is at least theoretically greater than 800 million pixels/second (800 MHz). The exposure time of the beam in SEEM may correspondingly be much longer than in SEM, and this permits a much lower current density while still maintaining a high Signal-to-Noise ratio. Thus, SEEM has the capability of investigating more sensitive sample surface structures while requiring lower brightness electron beam sources
Another approach is called Photo-Electron Emission Microscopy (PEM or PEEM), in which photons are directed at the surface of a sample to be studied, and by the photoelectric effect electrons are emitted from the surface. On an insulating surface, the emission of these electrons, however, may produce a net positive charge on the sample surface since there is a net flux of electrons from the surface. The sample continues to charge positively until there are no emitted electrons, or electrical breakdown occurs. This charge build-up problem may limit the utility of PEEM for imaging insulators. However, depending on the material of the sample, an opposite effect may occur in PEEM, namely that a surface conductance is generated by the incident photons. The mechanism underlying this effect is that of pair production of electrons and holes, so-called excitons, that are generated by the photons, thereby generating enhanced surface conductance of the sample. Thus, electrical charge may be transported away from the region of the incident light beam.
Another method of examining surfaces with electron beams is known as Low Energy Electron Microscopy (LEEM), in which a relatively wide beam of low-energy electrons is directed to be incident upon the surface of the sample, and electrons reflected from the sample are detected. However, LEEM suffers from a similar charge build-up problem since electrons are directed at the sample surface, but not all of the electrons are energetic enough to leave the surface. In LEEM, negatively-charged electrons accumulate on the surface, which repels further electrons from striking the sample, resulting in distortions and shadowing of the surface. In LEEM, it may pose a problem that the surface charges negatively due to the low electron energies.
TAB. 1 below is a chart summarizing the differences between, and advantages of, the four SEM, SEEM, PEEM and LEEM techniques:
TABLE 1Differences between, and advantages of, thefour SEM, SEEM, PEEM and LEEM techniquesSecondaryPhoto-LowScanningElectronElectronEnergyElectronEmissionEmissionEmissionMicroscopyMicroscopyMicroscopyMicroscopy(SEM)(SEEM)(PEEM)(LEEM)IncidentElectronsElectronsPhotonsLow-ParticlesEnergyElectronsDetectedSecondarySecondaryPhoto-ReflectedParticlesElectronsElectronsElectronsElectronsImagingRasterParallelParallelParallelMethodScanningImagingImagingImagingChargingLimitedLimitedPositiveNegativeChargingChargingChargeChargeBuild-UpBuild-Up
In Scanning Electron Microscopy (SEM), raster scanning imaging must be utilized which leads to a relatively low throughput because the electron beam is focused to a narrow spot size. SEM, however, produces energetic primary electrons incident at an energy of 0.5 to 1 keV, so that a relatively charge-neutral operation is attained. Energetic primary electrons produce secondary electrons in SEM.
In the Secondary Electron Emission Microscopy (SEEM) technique, a beam of energetic primary electrons is directed at the sample surface with an energy of about 0.5 to 1 keV. Because a relatively wide beam of primary electrons is introduced, parallel imaging becomes possible, which is significantly faster than SEM imaging. Moreover, the sample remains charge neutral.
PEEM uses photons instead of primary electrons to produce emitted secondary electrons. PEEM may suffer from the problem of positive charge build-up on insulating sample target materials because secondary electrons are being knocked off the sample surface by the photons, but no negatively charged particles replace these secondary electrons. The inspecting photon beam of PEEM can be wide, and parallel imaging can be achieved.
In Low Energy Electron Microscopy (LEEM), a wide beam of primary electrons is projected at the inspection surface, and parallel imaging can be achieved. These primary electrons are relatively low in energy, and the imaging method involves reflecting these low-energy electrons from the surface. Because only low energy electrons are incident, primary electrons are reflected but few secondary electrons are emitted. Also, the low energy implies a negative charge build-up because some of the electrons may be energetic enough to enter the sample but are not sufficiently energetic to escape the sample surface.